This invention relates to a tone waveshape generation device employed in an electronic musical instrument and, more particularly, to a device capable of reading out successive waveshapes of plural periods stored in a memory.
A device which prestores successive waveshapes of plural periods from the start to the end of tone generation in a memory and generates tone waveshape signals by reading out these prestored waveshapes is known, e.g., in the specification of U.S. Pat. No. 4,305,319. The United States patent discloses a drum generator in which snare drum sounds and other drum sounds are prestored in the form of successive waveshapes of plural periods in respectively corresponding memories and these waveshapes are read out in response to a sound timing signal (play strobe signal). This type of prior art tone waveshape generation device has heretofore been used as a tone source of rhythm sounds and increase in the memory capacity to some extent has not posed any serious problem since necessity for changing the tone pitch is not involved in this type of device.
If, however, this type of tone waveshape generation device is applied to generation of scale notes, it becomes necessary to prepare successive waveshapes of plural periods for respective different tone pitches or tone ranges with a result that the capacities of memories become extremely large. If, for example, the duration of tone generation is 5 seconds, the sampling period is 32 kHz and successive waveshapes of plural periods are prepared over 4 octaves one for each tone range which has been determined for 3 scale notes (keys) in 12 scale notes of one octave (totalling 16 tone ranges), a memory having a capacity of "32k.times.5.times.16=2560 kilo words" is required.
An electronic musical instrument of a type in which, in the above described manner, a complete waveshape from the start to the end of generation of a tone is prestored for each key (note) and then is read out is disclosed in the specification of U.S. Pat. No. 4,383,462. In the waveshape memory WM31 shown in FIG. 3 of this United States patent, a complete waveshape is stored and this complete waveshape is read out in response to a signal KD which represents a key depression timing.
An improvement has been conceived for preventing the capacity of the memory storing the complete waveshape from becoming too large. According to this improvement, the attack portion of the tone is stored in its entirety but only a part of the sustain portion is stored and the stored part of the sustain portion is repeatedly read out to generate the entire sustain portion. In the above U.S. Pat. No. 4,383,462, an example of such improvement is shown in FIG. 6. A complete waveshape in the attack period is stored in the waveshape memory WM61 and at least one fundamental period of a tone waveshape is stored in the waveshape memory WM62. An attack waveshape is read out from the memory WM61 in response to the key depression (KD signal) and the tone waveform of the fundamental period is repeatedly read out from the memory WM62 after completion of the read out of the attack waveshape (IMF signal) until the end of tone generation (DF signal). According to this improvement, the memory capacity can be reduced to, e.g. about one-fifth. If in this case the memory length of the memories corresponding to the respective tone pitches (tone ranges) is made uniform, blank area will occur in the memories. The memory length is determined by the lowest tone due to the fact that the rise time of the tone increases as the tone becomes lower and hence blank area will occur in a part of the zone of the memory storing the higher note waveshape which is shorter in the rise time resulting in the waste of the memory zone.